Exhaust gas recirculation (EGR) has proven to be very beneficial for fuel economy improvement as well as knock and emissions reduction. Combining with lean burning, it can help modern gasoline engines to become cleaner, more efficient and meeting the stringent emissions limit. However, there is a practical limit for lean mixture and EGR percentage for current engine due to many constraints, one of which being the ignition source. The Microwave Discharge Igniter (MDI), which generates, enhances and sustains plasma discharge using microwave (MW) resonance was tested to assess its ability in extending these limits. A combination of high-speed Schlieren imaging and pressure measurements were performed for propane-air mixture combustion inside a constant volume chamber to compare the dilution and lean limit between MDI and traditional spark plug. Nitrogen addition was carried out during mixture preparation to simulate the dilution condition of EGR. Results indicated that the MDI can successfully outperform the conventional spark plug and have better dilution and lean performance. Optimisation of MDI control parameters from its semiconductor source can further improve its performance in these conditions.